The present invention relates generally to electrostatic scan expansion lens systems for cathode-ray tubes, and more particularly to an improved mesh-type scan expansion lens for such tubes.
Cathode-ray tubes (CRT's) include an evacuated envelope comprising a tubular neck and a generally frustum-shaped funnel portion that is contiguous with one end of the neck and diverges outwardly therefrom. The outer end of the funnel portion is sealed against a face plate that carries a phosphorescent display screen.
An electron gun is positioned within the neck at the end opposite the funnel portion. The electron gun produces a beam of electrons that passes through the neck and funnel portion and illuminates a spot on the phosphorescent display screen. The beam also passes between two pairs of electrostatically charged deflection plates that are located in the neck between the electron gun and the display screen. The direction of the beam is deflected (hence, the position of the spot on the screen is changed) whenever a deflection voltage is applied to at least one of the pairs of deflection plates. The deflection voltage is continuously altered to deflect the beam so that, for example, a particular waveform is illuminated on the display screen. The amount of deflection of the beam for a given deflection voltage is known as deflection sensitivity. To increase the deflection sensitivity of a CRT is to increase the beam deflection without increasing the deflection voltage applied across the deflection plates.
The brightness of the spot illuminated on the display screen by the beam is characterized as the display luminance. The display luminance is increased by increasing the velocity of the beam, which is accomplished by increasing the beam accelerating voltage.
It is often desirable to construct a CRT with high display luminance and high deflection sensitivity. However, with conventional CRT designs, these performance goals usually conflict. Specifically, if the beam accelerating voltage is increased to raise the velocity of the beam before the beam passes the deflection plates, the deflection sensitivity of the beam decreases. That is, the beam is stiffer more resistant to deflection. This is so because deflection sensitivity is inversely proportional to the accelerating voltage. This conflict traditionally has been resolved by deflecting the beam in a region of low potential, then increasing the beam velocity by means of a high-voltage field after the beam exits the deflection region. This technique is commonly known as post-deflection acceleration, or PDA.
One type of PDA CRT creates the high-voltage field by placing an anode within the funnel portion of the CRT. Specifically, the anode comprises an electron-transparent conductive target layer overlying the display screen and an electrically connected continuous conductive film applied to the interior surface of the funnel portion. The electric field resulting from the presence of such an anode has increasing potential in the direction of beam travel and is, therefore, effective in accelerating the beam and increasing the display luminance. To enhance the deflection sensitivity of the CRT while simultaneously preventing penetration of the high-voltage field into the low-voltage deflection region, a field-forming mesh electrode is positioned within the tube between the deflection plates and the anode. The mesh comprises a multitude of interconnected webs forming an array of apertures. When incorporated into the CRT, the mesh has a concavo-convex configuration and is positioned with its convex surface facing the display screen. As a result, the equipotential surfaces of the high-voltage field generally conform to the convex shape of the mesh electrode. Since the forces created by the high-voltage field direct the electron beam to pass in a direction that is normal to the equipotential surfaces, the above-described force field created by the anode and mesh combination represents that of a diverging electron lens. That is, a beam passing through this field tends to diverge from the central longitudinal axis of the CRT. Accordingly, the beam divergence produced by this electron lens increases the deflection sensitivity of the CRT.
If the radius of curvature of the mesh electrode is reduced, the resulting curvature of the equipotential surfaces of the high-voltage field will cause correspondingly greater divergence of the beam. It can be readily appreciated that to achieve high deflection sensitivity, it is desirable to produce a mesh that is deformable into a concavo-convex shape having as short a radius of curvature as possible.
A mesh is typically formed by electrodeposition of metal (for example, nickel) onto a planar mandrel. The resulting planar mesh is then annealed. The concavo-convex shape is achieved by deforming the mesh within a curved mold. In the past, the mesh could be deformed by only a limited amount because too much deformation resulted in breakage of the fragile metal webs. Breakage results from tensile stresses that develop over the entire cross section of each web when the mesh is deformed. Metal, such as nickel, will strain (i.e., stretch) somewhat in response to the tensile stress but quickly reaches its tensile strength limit and breaks. As a consequence, the limited amount of curvature that could be formed into the mesh correspondingly limited the deflection sensitivity of the CRT into which the mesh was incorporated.